The present invention relates to a process for producing magnetic recording media. More particularly, the invention relates to a process for producing a multi-layered magnetic recording medium wherein a plurality of magnetic dispersions using an organic solvent are simultaneously applied in superposition onto a continuously running nonmagnetic support.
In the production of magnetic recording media, efforts have recently been made to increase the recording density or reduce the thickness of the magnetic layer. Under these circumstances, manufacturers are making a shift from a single to a multiple layer construction in the magnetic layer that is coated on a nonmagnetic support. This is because, compared to magnetic recording media having a single magnetic layer, media having multiple magnetic layers can be significantly improved in magnetic recording characteristics, as evidenced by an increased storage capacity of magnetic data. To obtain this advantage, the number of magnetic layers must be at least two or more.
Methods for achieving the purpose of forming multiple magnetic layers are described, for example, in Japanese Examined Patent Publications Nos. 54-43362 and 58-43816, as well as Japanese Unexamined Published Patent Applications Nos. 51-11920, 52-51908 and 53-16604. The basic principle of these methods is to apply individual coating solutions one by one onto a nonmagnetic support and dry the successively applied solutions to form multiple coated layers.
However, these methods, which require the repetition of coating and drying steps, are low in productivity, and the equipment required therefor is inevitably bulky, with the additional disadvantage of high equipment cost. Further, irregularities sometimes occur at the interface between adjacent coated magnetic layers, which can cause either frequent generation of unwanted tape modulating noise or inadequate adhesion between layers.
Accordingly, it has been desired to develop a method by which multiple magnetic layers can be formed in a single run of coating and drying steps. However, if the coating method for achieving simultaneous application of two or more layers in superposition described in Japanese Unexamined Published Patent Application No. 62-124631 is adopted to apply a nonmagnetic coating solution and a magnetic coating solution or two coating solutions simultaneously in superposition using an organic solvent, color unevenness or longitudinal streaks are more likely to occur, even at low coating speeds, than in the method that involves repeated performance of successive coating and drying steps. This causes quality deterioration in terms of both the characteristics of electromagnetic conversion and appearance. Further, depending on the combination of formulas of coating solutions, the coating solution for the upper layer cannot be coated uniformly to form two distinct layers or, alternatively, the coating solution for the lower layer. These problems have been found to become serious as the coating solutions are applied in smaller quantities (i.e., as they are applied in smaller thicknesses) or as they are applied at higher speeds.
If coatability at high speed is the only concern, high-speed coating can be accomplished by reducing the viscosity of the coating solutions. However, if the support has an uneven surface due to such defects as wrinkles, a low-viscosity (highly flowable) coating solution applied will flow under gravity before it is dried and move in such a way as to compensate for the irregularities in the surface, thereby causing variations in the thickness of the coated layer.
If the viscosity of the coating solution is reduced, coatability at small thicknesses at high speed could be sufficiently improved to enable application of layers in superposition to some extent. In practice, however, if the solvent content in the magnetic dispersion is increased to lower the viscosity of the dispersion, it will take a comparatively long time to evaporate the solvent and hence dry the applied magnetic layer. If the drying time is prolonged, due to its high viscosity, the magnetic material will settle to the bottom of the magnetic layer, thereby making it impossible for the magnetic layer to exhibit its inherent performance.
This phenomenon, commonly referred to as "sedimentation", can theoretically be prevented by shortening the drying time. If, to this end, intense heat is applied to evaporate the solvent rapidly, the rapidly evaporating solvent will produce convection in the coating solution, and the resulting turbulence can cause adjacent layers to mix with other or deteriorate the surface properties of the coating solution. Further, the increase in the speed at which the drying air is blown against the coated surface often causes the support to run erratically, and also introduces irregularities on the coated surface or at the interface between layers, thereby causing adverse effects on the various characteristics of the applied magnetic layers. Thus, even if coating of layers in superposition can be accomplished to some extent, the poor surface properties of the magnetic layers can cause certain problems in actual use of the magnetic recording medium as the final product; for example, if it is used a video tape, video characteristics will be impaired by large noise.
It is also known that when two comparatively viscous fluids having different compositions are superposed in layers, eddy convection will occur even if the fluids are merely superposed one on the other without applying any external force. In order to reduce the occurrence of this eddy convection, both the density and the surface tension of either one of the adjacent fluid layers may be made higher or lower than those of the other layer. The occurrence of eddy convection can also be prevented by increasing the viscosities of the individual fluids to very high levels. (See Kaisetsu Toryogaku, "An Introduction to the Science of Paints", K. Mihara, ed., p. 211, Riko Shuppan, 1975).
However, if the densities and surface tensions of the magnetic dispersions are limited within very narrow ranges, the latitude in the applicability of magnetic dispersions is reduced to impractical low levels. Further, as already mentioned, increased viscosities deteriorate the coatability in thin layers at high speeds and hence are not desired.
Methods of coating multiple layers are also employed in the production of photographic materials and thermosensitive papers. Coating solutions for producing photographic materials utilize the sol-to-gel conversion of gelatin that is caused by temperature adjustment, so that the viscosity of coating solutions can be easily set in such a way that they produce not only a sol state which is optimum for coating purposes but also a gel state which is suitable for drying purposes. If the support is paper as in the case of thermal paper, the water in a coated solution is absorbed by the paper even if the solution itself is fairly low in viscosity and, as a result, the viscosity of the coated solution is sufficiently elevated to prevent unwanted irregularities from occurring in the coated layer in subsequent stages. Thus, the physical properties of substantially Newtonian fluids as exemplified by coating solutions used in the production of thermosensitive papers and photographic materials are largely dependent on their static viscosity (which can be easily determined by formulas) and, as mentioned above, their viscosity can be adjusted fairly easily, thereby making it possible to apply multiple coatings simultaneously.
However, magnetic coating solutions used in the production of magnetic recording media are non-Newtonian fluids and their viscosity is not constant but varies under specific conditions. Hence, it has been extremely difficult to control the viscosity-related physical properties of these magnetic coating solutions, and hence previous attempts to solve the aforementioned problems by adjusting the viscosities of the solutions according to specific conditions have been empirical. Under these circumstances, it has been very difficult to produce multi-layered magnetic recording media with unconventionally higher consistency in quality and at higher production rates.